It is proposed to compare and contrast the development of normal full-term infants, uncomplicated prematures, and full-term infants with trisomy 21 syndrome, by means of quantitative analyses of longitudinal EEG and evoked potential data, to determine whether the experimental groups show lags and/or advancements in rates of electrophysiological development or other deviations, and to determine the optimal time during the early weeks of life for observing specific differences. Correlative aims include: (1) evaluation of the possible usefulness of interhemispheric amplitude symmetry as a developmental and/or clinical variable in the neonate, (2) further to investigate frequency characteristics of the newborn's EEG, and (3) to study the development of interhemispheric synchrony in the infant. Recordings are made weekly during the first 10 weeks post-term and then at 3, 6, 9, and 12 months in 10 selected subjects in each group. Quantitative longitudinal data on the early ontogenesis of human brain electrophysiological phenomena are obtained. These permit cross-validation of a number of reported observations, some of which may prove to be of clinical prognostic significance. Both the techniques used and the information obtained will be widely applicable in other studies of infant development.